1. Field of the Invention
The present invention relates to a serial printing apparatus. More specifically, the invention relates to an ink-jet apparatus controlling an ejection timing control.
2. Description of Prior Art
The recording apparatus can be generally classified into two kinds, i.e. serial type and full line type depending upon the configuration of a recording head. Among these, the serial type recording apparatus is to perform recording while the head is moving and is the type generally used. The most well known device of this kind is an ink-jet printing apparatus for printing characters and images and so forth by ejecting an ink toward a recording medium, such as a paper. Such ink-jet printing apparatus is typically used as an information output means of a printer, a copy machine, facsimile and so forth.
As a system that performs recording by ejecting the ink while the ink-jet head is moved, speed control for a carriage which mounts the ink-jet head and carries it, and an ejection timing control associated with motion of the carriage are important factors for determining a quality of a recorded image as the result of printing (hereinafter also referred to recording). More specifically, when the carriage is moved for recording (this moving is hereinafter also referred to as "scan"), the speed condition of the carriage transits in acceleration state, constant speed state and deceleration state. Normally, ink ejection is performed during the constant speed state. However, even in the constant speed zone, there is a slight fluctuation of the carriage motion speed. Therefore, it is desirable to restrict speed variation of the carriage during motion across the constant speed zone.
Such carriage speed control is frequently a closed loop control for driving the carriage motor, in which an output of an encoder detecting motion information of the carriage is used as a feedback signal. In this case, the encoder output is generally obtained from an optical or magnetic rotary encoder or linear encoder provided in a part of the ink-jet recording apparatus.
On the other hand, the ejection timing control is performed so that dots formed on the recording medium are arranged at a desired pitch by ejection of the ink in a scanning direction and determines the quality of recorded image in association with the carriage speed control. In the case that the carriage speed is controlled in closed loop, a detection signal of a linear encoder detecting the carriage motion information is used for controlling the ejection timing.
However, in the prior art set forth above, when the closed loop control of the carriage motor is performed, load on a CPU is increased for this process to result in lowering of a through-put of the overall apparatus due to lowering of the process speed in the CPU. When attempt is made to improve performance of the CPU for avoiding the problem of lowering of through-put, significant cost-up problem is concerned.
On the other hand, associating with increasing of density of ejection orifices of the recent ink-jet head, recording of image at higher resolution is becoming possible. In such case, the dot pitch in the scanning direction (an interval between ejection timings) has to be made smaller. In order to directly and easily realize this, increasing of resolution of the linear encoder can be considered. However, in order to increase resolution of the linear encoder, higher density of integrating of the elements and of scale becomes necessary to inherently cause a problem of cost-up.